A commonly misunderstanding of traceroute is that it fully relies on ping. “If I block ping at my firewall, no one can use traceroute to reveal my internal routing path”. Unfortunately this is not true. If traceroute is used with TCP SYN packets on permitted TCP/UDP ports, all intermediary firewalls will handle the IP packets with TTL = 0 corresponding to the RFCs and will reply with an ICMP time exceeded packet to the source.

In this post I am listing an example that uses traceroute with TCP port 25 (SMTP) to traverse a firewall. A sample pcap file can be downloaded while some Wireshark screenshots show a few details.